12. the Development of Calculus
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12. The development of calculus (Burton , 8.3 – 8.4 ) We have already noted that certain ancient Greek mathematicians — most notably Eudoxus and Archimedes — had successfully studied some of the basic problems and ideas from integral calculus , and we noted that their method of exhaustion was similar to the modern approach in some respects (successive geometric approximations using figures whose measurements were already known) and different in others (there was no limit concept , and instead there were delicate reductio ad absurdum arguments) . Towards the end of the 16 th century there was further interest in the sorts of problems that Archimedes studied , and later in the 17 th century this led to the independent development of calculus in Europe by Isaac Newton (1643 – 1727) and Gottfried Wilhelm von Leibniz (1646 – 1716) . Two important factors led to the development of calculus originated in the late Middle Ages , and both represented attempts to move beyond the bounds of ancient Greek mathematics , philosophy and physics . 1. Interest in questions about infinite processes and objects . As noted earlier , ancient Greek mathematics was not equipped to deal effectively with Zeno ’s paradoxes about infinite and by the time of Aristotle it had essentially insulated itself from its “horror of the infinite .” However , we have also noted that Indian mathematicians did not share this reluctance to work with the concept of infinity , and Chinese mathematicians also used methods like infinite series when these were convenient . During the 14 th century the mathematical work of Oresme and others on infinite series complemented the interests of the scholastic philosophers ; questions about the infinite played a key role in the efforts of scholastic philosophy to integrate Greek philosophy with Christianity . Philosophical writings by scholars including William of Ockham (1285 – 1349) , Gregory of Rimini (1300 – 1358) , and Nicholas of Cusa (1401 – 1464) provided insights which anticipated the concept of limit — a basic part of calculus as we know it . 2. Interest in questions about physical motion . During the 13 th century Jordanus de Nemore discovered the mathematically correct description for the physics of an inclined plane (a result that had eluded Archimedes and was described incorrectly by Pappus) , and later members of the Merton school in Oxford such as T. Bradwardine (1290 – 1349) and W. Heytesbury (1313 – 1373) had discovered an important property of uniformly accelerated motion ; namely , the average velocity (total distance divided by total time) is in fact the mathematical average of the initial and final velocities . The significance of this concept for the motion of freely falling bodies was not understood at the time and would not be known until the work of Galileo and others in the 16 th century . During the late 16 th century interaction between mathematics and physics began to increase at a much faster rate , and many important contributors to mathematics such as S. Stevin also made important contributions to physics (in Stevin ’s case , the centers of mass for certain objects and the statics and dynamics of fluids) . Here are some online references to further information about the topics from the Middle Ages described above : http://www.math.tamu.edu/%7Edallen/masters/medieval/medieval.pdf http://www.math.tamu.edu/%7Edallen/masters/infinity/infinity.pdf http://plato.stanford.edu/entries/heytesbury/ Problems leading to the development of calculus In the early 17 th century mathematicians became interested in several types of problems , partly because these were motivated by advances in physics and partly because they were viewed as interesting in their own right . 1. Measurement questions such as lengths of curves , areas of planar regions and surfaces , and volumes of solid regions , and also finding the centers of mass for such objects . 2. Geometric and physical attributes of curves , such as tangents and normals to curves , the concept of curvature , and the relation of these to questions about velocity and acceleration . 3. Maximum and minimum principles ; e.g ., the maximum height achieved by a projectile in motion or the greatest area enclosed by a rectangle with a given perimeter . Specific examples of all three types of problems had already been studied by Greek mathematicians . The results of Eudoxus and Archimedes on the first type or problem and of Archimedes and Apollonius on the second have already been discussed . We know about the work of Zenodorus (200 – 140 B.C.E.) on problems in the third type because it is reproduced in Pappus ’ anthology of mathematical works (the previously cited Collection or Synagoge ). Some examples of Zenodurus ’ results are (1) among all regular n – gons with a fixed perimeter , a regular n – gon encloses the greatest area , (2) given a polygon and a circle such that the perimeter of the polygon equals the circumference of the circle , the latter encloses the greater area , (3) a sphere encloses the greatest volume of all surfaces with a fixed surface area . GIVE REFERENCES Infinitesimals and Cavalieri ’s Principle We shall attempt to illustrate the concept of infinitesimals with an example of its use to determine the volume of a geometrical figure . Usually the formula for the volume of a cone is derived only for a right circular cone in which the axis line through the vertex or nappe is perpendicular to the plane of the base. It is naturally to ask whether there is a similar formula for the volume of an oblique cone for which the axis line is not perpendicular to the plane of the base (see the drawing below). One can answer this question using an approach developed by B. Cavalieri (1598 – 1647) . Similar ideas had been considered by Zu Chongzhi , also known as Tsu Ch ’ung – chin (429 – 501) , but the latter ’s work was not known to Europeans at the time . CAVALIERI’S PRINCIPLE. Suppose that we have a pair three-dimensional solids S and T that lie between two parallel planes P1 and P2, and suppose further that for each plane Q that is parallel to the latter and between them the plane sections Q ∩∩∩ S and Q ∩∩∩ T have equal areas . Then the volumes of S and T are equal . Here is a physical demonstration which suggests this result : Take two identical decks of cards that are neatly stacked , just as they come right out of the package . Leave one untouched , and for the second deck push along one of the vertical edges so that the deck forms a rectangular parallelepiped as below . In this new configuration the second deck has the same volume as first and it is built out of very thin rectangular pieces (the individual cards) whose areas are the same as those of the corresponding cards in the first deck . So the areas of the plane sections given by the separate cards are the same and the volumes of the solids formed from the decks are also equal . We shall now apply this principle to cones . Suppose we have an oblique cone as on the right hand side of the figure below . On the left hand side suppose we have a right circular cone with the same height and a circular base whose area is equal to that of the elliptical base for the second cone . Figure illustrating Cavalieri’s Principle In the notation of Cavalieri ’s Principle , we can take P1 to be the plane containing the bases of the two cones and P2 to be the plane which contains their vertices and is parallel to P1. Let Q be a plane that is parallel to both of them such that the constant distance between P2 and Q is equal to y; we shall let h denote the distance between P1 and P2, so that h is also the altitude of both cones . If b denotes the areas of the bases of both cones then the areas of the sections formed by intersecting these cones with Q 2 2 are both equal to b y / h . Therefore Cavalieri ’s Principle implies that both cones have 2 the same volume , and since the volume of the right circular cone is b h / 3 it follows that the same is true for the oblique cone . One approach to phrasing our physical motivation in mathematical terms is to imagine each cone as being a union of a family of solid regions given by the plane sections ; for the right circular cone these are regions bounded by circles , and for the oblique cone they are bounded by ellipses . Suppose we think of these sections as representing cylinders that are extremely thin . Then in each case one can imagine that the volume is formed by adding together the volumes of these cylinders , whose areas — and presumably thicknesses — are all the same , and of course this implies that the volumes of the original figures are the same . One fundamental question in this approach is to be more specific about the meaning of “extremely thin .” Since a planar figure has no finite thickness , one might imagine that the thickness is something less , and this is how one is led to the concept of a thickness that is infinitesimally small. The preceding discussion suggests that an infinitesimal quantity is supposed to be nonzero but is in some sense smaller than any finite quantity . If we are given a geometric figure that can be viewed as a union of “indivisible ” objects with one less dimension — for example , the planar region bounded by a rectangle viewed as a union of line segments parallel to two of the sides , or the solid region bounded by a cube as a union of planar regions bounded by squares parallel to two of the faces — then the idea is to view the square as a union of rectangular regions with infinitesimally small width or the square as a union of solid rectangular regions with infinitesimally small height.